Japanese Patent Application Laid-open Publication No. 2001-85591 (Patent Document 1) describes that a groove is formed in an upper surface of a die pad (chip pad) on which a semiconductor chip is mounted.
Also, Japanese Patent Application Laid-open Publication No. 2008-211231 (Patent Document 2) describes that a die pad is deformed by metal stamping so that a circular central part thereof on which a semiconductor chip is mounted protrudes relative to its surrounding part.
In addition, Japanese Patent Application Laid-open Publication No. 2002-134677 (Patent Document 3) describes a structure in which a thin portion obtained by removing a part of a lower surface by etching is provided in the vicinity of the peripheral edge of a die pad (island) on which a semiconductor chip is mounted and a slit is formed in the thin portion.